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Noxious weeds

King County, Washington

To offer a suggestion or report an error on the King County Noxious Weeds website, please contact Sasha Shaw, education specialist.

Invasive knotweeds
Polygonum x bohemicum, P. cuspidatum, P. sachalinense, P. polystachyum
Buckwheat Family

Invasive knotweed control videos video

General description

Invasive Knotweed on the Raging River - click for larger image In the Pacific Northwest, there are four similar species of invasive knotweed that are difficult to tell apart and share similar habitat, impacts and control methods. They are all large, robust perennials that spread by long creeping rhizomes to form dense thickets. Knotweed stalks - click for larger imageThese tall, bamboo-like plants were introduced from Asia as ornamentals beginning in the early 1800's in England and in the United States by 1890.

Due to their widespread use, the lack of natural predators, and their ability to spread by root and stem fragments, invasive knotweed species have spread and become widely established throughout North America and Europe. Knotweed clones can completely clog small waterways and displace streamside vegetation, increasing bank erosion and lowering the quality of riparian habitat for fish and wildlife. Rapid spring growth and deep, extensive roots enable knotweed to outcompete most other plants, even small trees and shrubs. Knotweeds can tolerate partial shade and are most competitive in moist, rich soil. Invasive knotweed species are commonly found along roadsides and on stream banks but also may be present in yards, vacant lots, edges of fields, parks and many other places.

Legal Status

Public and private landowners are not generally required to control infestations of invasive knotweed that occur on their property in King County, Washington, except in selected areas on the Green River and its tributaries and on the Cedar River and its tributaries, as described on the King County Weed List. Invasive knotweed species are all Class B Noxious Weeds in Washington. They have not been designated for required control in the county by the Washington State Noxious Weed Control Board, but they have been selected for required control in limited parts of the county by the King County Noxious Weed Control Board. Because control of these species is not generally required in the county, they are on the list of Non-Regulated Noxious Weeds for King County. For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.

These species are also on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of these species, into or within the state of Washington.

Identification

Stems are stout, cane-like, and reddish-brown. The plants die back at the end of the growing season but their old reddish-brown canes often persist. The stem nodes are swollen and surrounded by thin papery sheaths. Leaves are either heart-shaped or spade-shaped or somewhere in between. The flowers are small, creamy white to greenish white, and grow in showy plume-like, branched clusters from leaf axils near the ends of the stems. The fruit is 3-sided, black and shiny.

County-led knotweed control projects

Control of knotweed is challenging and often requires a watershed approach to be effective along waterways. The King County Noxious Weed Control Program is engaged in several cooperative weed managment projects in the county to tackle this difficult weed along certain waterways with assistance from local landowners and state and federal grants. For information on these projects, please see our Reports page or contact our office and ask to speak with the Knotweed Program Manager.

For more information Stop the Spread of Invasive Knotweed - click to download

Invasive Knotweed Brochure (2 Mb)
Himalayan Knotweed Fact Sheet (156 Kb)
Knotweed Best Management Practices (281 Kb)
Knotweed Weed Alert (215 Kb)
Knotweed Biology and Control Slide Show (Note: pdf format, 7.09 Mb)
Written Findings of the WA State Noxious Weed Control Board on Japanese and Bohemian knotweed (external links)

Knotweed photos - click a thumbnail for a larger image

Knotweed root crown - click for larger image Knotweed on fallen tree in river - click for larger image Flowering knotweed on hillside - click for larger image Knotweed flowers - click for larger image Knotweed leaves - click for larger image
Knotweed on Middle Fork Snoqualmie River - click for larger image Knotweed clump in river - click for larger image New growth on knotweed - click for larger image Knotweed stem - click for larger image Top of knotweed stem - click for larger image
Knotweed by river - click for larger image Knotweed on Cedar River - click for larger image Knotweed in Cedar River - click for larger image Cedar River with knotweed - click for larger image Knotweed at Cumberland Kanaskat - click for larger image
Knotweed by Lake Washington - click for larger image Brush cutting knotweed - click for larger image Knotweed sprout - click for larger image Knotweed new leaves - click for larger image Knotweed by Beckler River Rd - click for larger image

Related information

Related agencies


Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).